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Converting the Mole

Recipes for cooking or baking generally use units such as cups and tablespoons. These common units allow us to be able to compare the quantities of different ingredients and accurately produce a correct quantity of the resulting food item.

In chemistry we are also concerned with using common units in “recipes”. The recipes that are used in chemistry are called chemical equations. These chemical equations describe the quantities of products and reactants in a chemical reaction (more on that in the lesson, Balancing Chemical Equations). But what is the common unit in a chemistry recipe? We don’t use cups, tablespoons, litres or even grams. In chemistry the common unit is the Mole.

Recall the the mole is a unit that similar to the dozen. It allows us to compare quantities in a chemical reaction and predict the quantities of product we can produce.

You may have noticed however, that there are no instruments used in the chemistry lab that give measurements in moles. In the chemistry lab we commonly use balances that measure in grams and graduated cylinders that measure in millimetres. Since we so commonly use grams and litres in the lab we must have a way to convert from those units into the mole

Converting Between Grams and Moles

If you had a bag of pennies and I asked you to tell me how many pennies are in that bag could you do it? Sure you could, you would simply dump out the pennies and count them up. Now what if I told you that the pennies must remain inside the bag and that you could not simply just count them. Believe it or not, you could still accurately determine the number of pennies in the bag and you would only require two pieces of information.

If I were to tell you that a single penny has a mass of 2.35 grams and that the bag of pennies has a mass of 183.3 grams (we are going to assume that the bag itself doesn’t have any mass), could you think of a way to determine the number of pennies in the bag? Of course you can! If you know the total mass of all of the pennies and you know the mass of a single penny you can determine the number of pennies in the bag by using the following calculation,

Figure 1

We use this exact same method in chemistry. Rather than using the mass of a single atom, we use the mass of a single mole of atoms. If we look on a periodic table, we will see that these values are already listed for us! The Molar Mass of an element is the mass of one mole of atoms for that particular element. So what is the mass of one mole of carbon atoms? If we look on the periodic table we can see that the molar mass of carbon is 12 g/mol, so the mass of one mole of carbon atoms is 12 g!

Example 1: Converting between grams and moles

Example 2: Converting between grams and moles

What if you already have grams and you want to convert to moles? We follow the same two steps as before, however this time we will divide by the molar mass of the compound. Or in other words, we will multiply by the inverse of the molar mass (just flip the fraction).